1. Field of the Invention
The present invention relates to processes and compositions for non-destructively testing surfaces of materials to locate defects, and is specifically adapted to processes for determining the surface integrity of fuel tanks without the introduction or use of an electrically powered or "hot" light source.
2. Background of the Invention
One of the areas of increasing concern in the aerospace industry today is the integrity of fuel tank structures. Often, during or after flight, surface defects, such as cracks and holes, are discovered. Inspection of the tank surfaces, both inside and out, must be undertaken in order to locate these defects.
Penetrants incorporating a dye component have long been known and used to detect surface defects, such as cracks in workpiece surfaces. Examples of such penetrant compositions are disclosed in U.S. Pat. Nos. 4,392,982, 4,186,304 and 3,915,886.
Typically, dye penetrant is applied to the surface to be inspected in liquid or solid form, after which the surface is superficially cleaned to remove excess dye and to leave only residual dye on minute quantities which are retained within the surface voids or defects. Depending on the composition of the penetrant, an emulsifying agent or solvent is sometimes used to transform excess penetrant as necessary for solubility of the penetrant in the cleaning solution. Cleaning is often done by scrubbing the surface with a sponge or cloth saturated with the emulsifying agent or cleaning solution, or both.
In the past, the dye used in the inspections consists of a fluorescent material which Is applied to the surface to be inspected. The inspections then must be carried out using a "hot" light, as for example an electrically powered ultraviolet or fluorescent light source. Once application of the penetrant has been appropriately carried out, it is possible to determine the location and nature of surface defects or flaws by directing the "hot" light onto the treated surface and noting the areas of the surface where the penetrant material is visible.
Quite understandably, there is much concern for human safety when an inspection of fuel tanks is performed with hot light, since almost all aerospace fuel vapors are extremely volatile and explosive. Moreover, innumerable safety precautions must be taken or standards complied with in order to protect against explosion of the fuel and consequent damage to property or loss of life. For these reasons, as well as the desire to eliminate the inordinately high cost associated with the equipment and procedures used in carrying out these inspections, it has become most desirable to find alternative flaw detection/inspection methods and/or apparatus for use in environments where volatile substances have been contained, so that the danger of fire or explosion is substantially eliminated.